Variations in the static droop law
Whilst of obvious benefit to aircraft operation the installation of collective anticipation introduces a complication in the documentation of static droop as the datum RRPM is now dependent on collective position. However, the power required by the main rotor, although similarly dependent on collective pitch, is a function of airspeed, vertical speed and yaw pedal deflection/sideslip. Therefore it is possible for the stable RRPM to change with airspeed or ROC even though the collective position is unchanged. This phenomenon is best explained by example. Consider the variation of power required with airspeed for a typical conventional helicopter (Fig. 6.10). The ‘power bucket’ can be clearly seen as can the fact that at high forward speed the hover
power can be exceeded. The variation of power can be translated into the collective position required for level flight. So even in level flight it is clear that the rotor speed will vary due to changes in the NR datum as the collective lever is raised and lowered.